Today we all celebrate April Fools' Day, sometimes called All Fools' Day. Its origins are uncertain. Some see it as a celebration related to the turn of the seasons, while others believe it stems from the adoption of a new calendar, when New Year's Day was moved from March 25-April 1 to January 1.
Some people were only informed of that change several years later and continued to celebrate New Year on the last day of the former celebration, April 1. And these people were labeled "fools".
It's also interesting, that in France, for example, they call April 1 "Fish Day", because people stick paper fish on each others backs.